This invention relates to mobile radiotelephone communications systems and, in particular, to systems of this type adapted to interface with standard telephone equipment.
In present day mobile radiotelephone communications systems, subscribers to a system utilize radiotelephones provided with radiotelephone transceivers to transmit and receive radio signals carrying calls from and to the subscribers. These calls are coupled through base stations which serve their respective coverage area of the radiotelephone system.
The radiotelephone system also includes a radiotelephone switching system referred to as a mobile telephone switching office (MTSO) or a mobile switching center (MSC) which radio communicates with and provides selective interconnection of calls to and from the base stations. The MTSO is additionally linked via lines or trunk circuits to the standard public or private switched telephone network (PSTN). As a result, subscribers of the mobile system using their radiotelephones can communicate with subscribers on the PSTN using their standard telephone stations, i.e., dial pulse or dual tone multi-frequency (DTMF) stations.
In a system of this type, a call from a subscriber on the radiotelephone system to a subscriber on the PSTN is routed from the radiotelephone of the calling subscriber to a base station. The radio base station then communicates the call to the MTSO which directs the call to the PSTN. The latter network then carries the call to the called subscriber.
The reverse process occurs when a subscriber on the PSTN places a call to a called subscriber on the radiotelephone system. In this case, the call from the calling subscriber is conveyed to the PSTN which relays the call to the MTSO. The latter office then passes the call to the appropriate base station which then communicates the call to the radiotelephone of the called subscriber. The MTSO thus has the following two points of interface: (1) inter-office trunks that interface to the public switched telephone network; and (2) base stations which interface with the radiotelephones used to provide the radio signal connections.
As can be appreciated, with the aforesaid system, subscribers using standard telephone stations and standard telephone equipment can only access the radiotelephone system through the PSTN in order for a call to be completed. However, in certain circumstances, it may be desirable for such subscribers to be able to gain direct access to the radiotelephone system. Direct access to the radiotelephone system might be beneficial where there is a need to provide back-up protection against disruption of the PSTN, or in areas where the PSTN has not as yet been made available.
U.S. Pat. No. 4,922,517 discloses one technique for allowing this direct access. In the '517 patent, an interface circuit provides an interface between a standard two-wire dial pulse or DTMF telephone station and a radiotelephone transceiver. The interface circuit of the '517 patent simulates to the telephone station the standard two-wire line or loop circuit used in the telephone switching system of the PSTN.
In the PSTN, the standard two-wire line circuit and its associated telephone station correspond to a specific directory or telephone number on a one-to-one basis. A call originating or terminating at the telephone station having this directory number will thus be handled by the associated line circuit of the telephone switching system.
When a call is terminated at a telephone station, the telephone switching system via the line circuit signals the telephone station of the incoming call by applying an alternating current ringing signal to the line circuit. This signal rings the bell typically found within the telephone station. On the other hand, when a call is originated at the telephone station, the telephone switching system supplies to the telephone station via the line circuit "dial tone" audio to signal the telephone station to forward routing information (i.e., the directory or telephone number being called). This information is forwarded using DTMF tones or dial pulse signaling, i.e., interruptions in the loop current, depending upon the telephone station type, i.e., DTMF or dial pulse.
A telephone station originating a call signals the call origination to the telephone switching system by going off-hook. This results in an electrical continuity or discontinuity condition in the line circuit. The telephone switching system detects this condition by the presence or absence of current flow and initiates return dial tone audio as above-mentioned.
The above interactions between a standard telephone station and a standard telephone line circuit of a telephone switching system include both supervision for call origination and termination and signaling for alerting users and for transfer of routing information. Supervision and signaling for a line circuit can be summarized from the above as follows: supervision is provided by the presence and/or absence of line current (typically loop start signaling or ground-start signaling); signaling is provided to the telephone station from the switching system for terminating calls by alternating current ringing signals; start dial signaling is provided by the switching system to the telephone station for originating calls by dial tone audio; answer supervision is provided by the switching system to the telephone station for terminating calls by removal of the alternating current ringing signals; and signaling is provided by the telephone station to the switching system for routing information by dial pulse or DTMF signals.
As above-indicated, the interface circuit of the '517 patent enables direct access to a cellular system only via the above-discussed standard telephone line circuit. However, this limits the usefulness of the interface, since it cannot be used with standard telephone interoffice telecommunications facilities.
A standard interoffice telecommunications facility is normally not associated with a specific telephone station or telephone number. Instead, such a facility is shared by numerous telephone numbers and telephone stations on an availability basis. Accordingly, the telecommunications facility is customarily referred to as a "trunk circuit", since it sequentially aggregates telephone calls between discrete telephone switching systems, involving the telephony communications of numerous originating and terminating directory or telephone numbers.
Once a telephone switching system has received from a telephone station routing information (i.e., a directory or telephone number) over a standard telephone line circuit, it may be necessary to pass the call to another telephone switching system in order for the call to reach its ultimate and correct destination. If this is the case, the originating-end switching system will typically seize an idle trunk circuit serving the two switching systems.
When a trunk circuit is seized at the originating-end switching system, seizure is communicated to the receiving-end switching system. This occurs generally using E&M "out of band signaling" and supervision established for the trunk circuit at the originating-end switching system or via labeled messages in the Common Channel Signaling (CCS) system, Signaling System 7 (SS7), Integrated Services Digital Network (ISDN) or similar communication facility. The receiving-end switching system will also establish similar signaling and supervision for the trunk circuit to advise the originating-end switching system when to forward any routing information. The same trunk circuit sequence is used regardless of which direction the call delivery is to take and is at a system to system (machine-to-machine) communication level. This is unlike the standard telephone line circuit signaling and supervision which is primarily intended for machine to human interaction.
For trunk circuits, the above-discussed supervision and signaling can be summarized as follows: supervision is provided by E&M lead signaling, loop reverse battery signaling (RV-O, RV-T), duplex signaling (DX), single frequency signaling (SF), digital multiplexed signaling or labeled messages; signaling is through multi-frequency R1 (MF), multi-frequency R2 (MF-R2), DTMF, dial pulse or labeled messages; start dial signaling from the switching system is via a temporary supervision state change (referred to as a "wink") or labeled messages; and answer supervision from the switching system is provided by a supervision state change or labeled messages.
As above-noted, present day telephone system equipment can only provide direct access to a radiotelephone system via a standard telephone line circuit. It would be desirable, however, to also be able to provide direct access through a standard trunk circuit connected to a telephone switching system.
It is, therefore, an object of the present invention to provide an interface circuit for permitting a standard telephone trunk circuit to directly access a radiotelephone communications system.
It is a further object of the present invention to provide an interface circuit which interfaces a standard telephone trunk circuit with a transceiver of a radiotelephone of a radiotelephone communications system.
It is yet a further object of the present invention to provide a radiotelephone communications system adapted to utilize an interface circuit meeting the above-stated objectives.
It is still a further object of the present invention to provide a routing correlator in a radiotelephone communications system meeting the above-stated objective which cooperates with the MTSO or MSC of the radiotelephone system to route calls to the radiotelephone, interface and interfaced trunk circuit of the system.